Rollup merge of #109793 - lukas-code:negative-impl-comment, r=scottmcm

add comment to `impl !Error for &str`

I saw this impl in the [standard library docs](https://doc.rust-lang.org/nightly/core/error/trait.Error.html#impl-Error-for-%26str) and wondered why it's needed. This commit adds a comment to explain its existence.

library/core/src/str/mod.rs

@@ -2655,5 +2655,6 @@ struct UnsafeBytesToStr impl<'a> Fn = |bytes: &'a [u8]| -> &'a str {
     };
 }
 
+// This is required to make `impl From<&str> for Box<dyn Error>` and `impl<E> From<E> for Box<dyn Error>` not overlap.
 #[stable(feature = "rust1", since = "1.0.0")]
 impl !crate::error::Error for &str {}